Spark erosion device having arc detecting means

ABSTRACT

Described is an apparatus for the timely detection of arcs during a spark erosion process. The breakdown delay time upon each discharge is compared to a reference time interval. In the event of a number of successive discharges with breakdown delay times shorter than this time interval, an error signal is produced.

United States Patent [191 De Bont et a1.

[451 May 21, 1974 SPARK EROSION DEVICE HAVING ARC DETECTING MEANS Inventors: Paulus Maria De Bont, Emmasingel,

Eindhoven; Willem Van Liefland, Nijmegen, both of Netherlands Assignee: U.S. Philips Corporation, New

York, NY.

Filed: Nov. 29, 1972 Appl. No.: 310,647

Foreign Application Priority Data Dec. 8, 1971 Netherlands 7116823 US. Cl 219/69 P, 219/69 C Int. Cl 823k 9/16 Field of Search 219/69 G, 69 C, 69 P, 69 S [56] References Cited UNITED STATES PATENTS 3.632.942 1/1972 Kondo 219/69 G Primary Examiner-.1. V. Truhe Assistant Examiner-Hugh D. Jaeger Attorney, Agent, or Firm-Frank R. Trifari 5 7] ABSTRACT 12 Claims, 4 Drawing Figures 33 34 n as 41 3's J L PATENTEUHAY 2 1 I974 SHEET 2 BF 2 SPARK EROSION DEVICE HAVING ARC DETECTING MEANS The invention relates to apparatus for the timely detection of arcs during a spark erosion process in which a supply source connected across a spark gap formed between a workpiece and an electrode tool produces discharges across the spark gap.

When removing particles of material from electrically conductive workpieces by means of spark erosion, spontaneous arcing may occur under conditions in which the flow of the dielectric fluid through the sparkgap is substantially impeded. Such an arc consists of a number of discharges with fixed locations. An arc is particularly likely to occur when the electrode tool consists of graphite. The are, which continuously restrikes in the same place, burns a deep hole into the workpiece. In the event of late detection of the existence of such local discharges the workpiece is damaged to such an extent that it is often spoiled. Hence, it is of great economic importance to be able to detect arcs at an early stage in order to enable the spark erosion process to be adjusted for an optimum result.

ln the French Pat. application No. 2,010,484 laid open to public inspection, there is described a device for the detection of arcs which measures whether, in a certain time interval, the voltage across the workpiece and the electrode tool has been higher than a reference value. The said device utilizes the fact that a spark discharge occurs at a voltage across the spark gap which is much higher than the voltage existing across the spark gap when an arc discharge occurs. The voltage across the spark gap is compared with a reference voltage from a zener diode, which diode controls a transistor. This transistor is included in the discharge circuit of a capacitor which is charged with a constant current. Should a number of consecutive pulses develop across the spark gap which are not accompanied by a spark discharge, the voltage across the capacitor will exceed a given value and a control signal will be applied to the supply pulse generator. One spark discharge across the spark gap causes the capacitor to be fully discharged.

It is an object of the invention to provide an apparatus for the detection of arcs which is based on an event associated with the occurrence of arcs and hitherto not employed for the detection of those arcs. The apparatus according to this invention is characterized in that electronic means are provided for comparing the breakdown delay time upon each discharge with a reference time interval, as well as a counting circuit for counting the number of consecutive discharges having breakdown delay times shorter than the reference time interval, and a comparator circuit which produces an error signal if this number is greater than a reference number. The breakdown delay time is defined as the time elapsing between the application of a voltage of a sufficiently high level to enable a discharge to be produced, which is referred to as open-circuit voltage or no-load voltage, and the occurrence of the discharge. The invention is based on the recognition that the disappearance of the breakdown delay time is a very useful criterion allowing the occurrence of a stationary arc to be detected in due time. Whereas the breakdown delay time in the case of the occurrence of discrete sparks of changing location, the desired condition, is a constantly varying but always measurable value, the

initiation of arcs is always accompanied by a series of breakdown delay times approximating to zero.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which:

FIG. 1 schematically shows a conventional power supply unit for spark erosion apparatus;

FIG. 2 shows the pulses supplied by this power supply unit;

FIG. 3 shows a preferred embodiment of an arc detection apparatus according to the invention; and

FIG. 4 shows the signals which are produced at various points in this apparatus.

In the apparatus according to FlG. l, a workpiece 2 is connected to the positive terminal of a current source 3 via a switching transistor 4 and an impedance 6. The electrode tool 1 is connected to the negative terminal of the aforementioned current source. As an alternative the electrode tool may be connected to the positive and the workpiece to the negative terminal of the current source. The presence of an electric current through the spark circuit is detected with the aid of a current discriminator 7. Two monostable multivibrators supplying pulses of constant pulse duration are designated by reference numerals 8 and 9. An amplifier 10 transfers these pulses to the switching transistor 4 in the spark circuit.

In FIG. 2 the voltage U, between the workpiece and the work electrode is plotted as a function of .time t. The no-load voltage U,,, which is forinstance volts, is the maximum voltagewhich can occur across a spark gap. After a time interval, which is referred to as the breakdown delay time T,,, after the voltage U, has occurred across the spark gap, a discharge is produced in the spark gap. During this discharge the voltage between workpiece and electrode tool drops to the socalled operating voltage U,,, which is, for example, 20 volts.

At the instant at which a current of a certain value starts flowing in the spark circuit, the current discriminator 7 supplies an output signal by means of which the monostable multivibrator 8 is controlled. At the output of this monostable multivibrator there appears a pulse P, with a duration T,. This pulse determines how long transistor 4 remains conductive and, consequently, how long power is supplied to the spark gap. During a time interval T, a current flows through the spark circuit. After the time interval T, the pulse at the output of monostable multivibrator 8 disappears and, simultaneously, i.e., on the trailing edge of pulse T, monostable multivibrator 9 is triggered. At the output of this monostable multivibrator a pulse P, with a duration T appears. This pulse, which is of a polarity opposite to that of pulse P,, determines how long transistor 4 will remain cut off. For a time interval 'l, the voltage across the spark gap is 0 volts. Subsequently, the noload voltage reappears across the spark gap and after a time interval T the breakdown delay time, the next discharge develops across the spark gap.

Although the breakdown delay time is not a constant parameter, but varies with process conditions, it always has a measurable value. This delay time can vary from approx. 0.1 usec to, for example, 500 usecs. During the initiation of a stationary are, however, the breakdown delay time always decreases to a very low value (T 0.l used). This property can be utilized according to the invention in order to obtain a reliable and rapid arc detection. For this purpose the breakdown delay time before each discharge is measured, discharges for which this delay time exceeds a certain value, for example, 1 to 2 usecs, being accepted and discharges with shorter delay times being rejected.

For a spark erosion operation which proceeds in a normal and desired manner a delay time limit can be indicated above which most of the delay times are situated. The delay times are distributed around this limit in such a way that one or two delay times which are shorter than the limit are followed by delay times which are longer than the limit. Consequently, in a normal spark erosion process, 1 or 2 unacceptable discharges are followed by a number of acceptable discharges. It has been found that in the case of an initiation to arcs a series of consecutive discharges occur for which the ignitiondelay time'approximates to sec, i.e., is shorter than the limit. Hence, the apparatus according to the invention is designed so that only when a series of discharges with a vanishingly small breakdown delay time occur, an indication is provided that arcs are likely to occur. 1

FIG. 3 shows an embodiment of a detection apparatus according to the invention, and FIG. 4 illustrates how the signals are processed in the apparatus according to FIG. 3.

As an example four successive discharges are shown in FIG. 4a, the limit of the breakdown delay time being setso that the second discharge is found to be acceptable, whereas the first, the third and the fourth discharge are found to be unacceptable. FIGS. 41; and 40 represent the pulses P, and P, corresponding to these discharges.

As is shown in FIG. 3, two trains of pulses P, and P, are applied to the inputs 31 and 32 of the apparatus. The circuit of pulse P, includes a delay element 33 which stretches pulse P,,v in other words, it delays the trailing edge of pulse P, relative to pulse P,. After element 33 a pulse P,, is obtained (see FIG. 4d). The delay of element 33 is adjustable, for example, between 0 and 2 psecs. Pulse P,, is applied to an input of a logic gate 34, and pulse P, is applied to the other input of this gate. At the output of this gate a counting pulse appears if a signal is present at both inputs, i.e., if the leading edge of P appears prior to the disappearance of the delayed trailing edge of the pulse P The resulting counting pulses, represented in FIG. 4e, are fed to an electronic counter 38. In a comparator circuit 39, the content of this counter is compared to the content of element 40, which element indicates the number of incorrect discharges to be produced successively before corrective action is to be taken. Element 40 can be set between 1 and 99.

If the number of counting pulses exceeds the preset number, comparator circuit 39 supplies an output signal. This signal can be used to control the power supply unit so as to suppress arcing. For this purpose, the output signal of comparator circuit 39 can, for example, control a monostable multivibrator 42, which supplies a fairly long pulse, adjustable between 1 and 99 msecs. This pulse P, can be applied to the monostable multivibrator 9 of the power supply unit where it overlaps pulse P, so that an extended pause interval T, is obtained. As a result, the power current is interrupted for an extra long time. When increasing the distance between workpiece and electrode tool, this will permit a better removal of eroded-particles. Moreover, the area of the disturbance is allowed extra time to cool down.

. P are applied, which have been obtained by passing pulses P, and P through inverter circuits 36 and 37.

For an acceptable discharge, i.e., a discharge at which the leading edge of pulse P, does not appear until after the delayed trailing edge of pulse P,, a so-called reset pulse is produced at the output of gate 35 (See FIG. 4f). This reset pulse is applied to the reset input 41 of the counter 38, thus resetting the counter to zero. The counter is also reset to zero when the discriminator circuit 39 supplies an output pulse, i.e., when the counter has counted a number of unacceptable discharges equal to the number preset in element 40. FIGS. 4g and 4h show the signal P from the comparator 39 and the reset pulse P for the case where element 40 is set to 2.

For simplicity, the apparatus according to the invention shown in FIG. Sand described above has been represented only in block-schematic form. The choice of elements which can-perform the functions of the blocks shown in FIG. 3 will'not present any problems to a person skilled in the art.

The are detection apparatus according to the invention is explained in conjunction with the special power supply unit shown in FIG. 3 because the pulses P, and P, supplied by the multivibrators can be used directly to control the arc detection apparatus. Obviously, the field of application of the apparatus according to the invention is muchwider, namely any spark erosion process permitting the detection of the instant at which the no-load voltage and the operating voltage occur, and the instant at which the power supply to the spark gap ceases.

What is claimed is:

1. A spark erosion apparatus with means for detecting an are between a workpiece and an electrode during a spark erosion process comprising, a source of supply voltage connected across the spark gap formed between the workpiece and electrode for producing electric spark discharges across said gap, means for effectively comparing the breakdown delay time of each gap electric discharge with a reference signal having a reference time period that defines the transition between a normal spark discharge and an arc discharge to produce an output pulse for each gap discharge with a breakdown delay time shorter than said reference time period, means for counting the number of consecutive output pulses corresponding to gap discharges having breakdown delay times shorter than the reference time period, and means for comparing the number of pulses counted with a reference number to produce an output signal when the count exceeds the reference number.

2. Apparatus as claimed in claim 5 wherein the sequence of pulses with a pulse width corresponding to the time interval during which an electric discharge occurs across the spark gap, a delay element for extending the width of the pulses of said first pulse sequence, and means for applying the delayed first pulse sequence to the gate first input and the second pulse sequence to the gate second input.

3. Apparatus as claimed in claim 6 further comprising means jointly responsive to the delayed first pulses and the second pulse sequence for deriving a reset signal when the breakdown delay time is greater than the reference time period, and means for applying said reset signal to a reset input of the counting means.

4. Apparatus as claimed in claim 7 wherein said reset signal deriving means comprises a second logic gate with first maseasnamsmsi first 553' sesdndwsepalarity inverter circuits interposed in circuit between the output of the delay element and the second gate first input and between the output of said second pulse dgriying means and the second gatesecond input, respectively, and means for coupling'the output of the second logic gate to said counting means reset input.

5. Spark erosion apparatus with means for detecting an arc in the gap formed between a workpiece and an electrode comprising, a source of supply voltage for producing electric spark discharge pulses across the spark gap, means for selectively connecting the supply source across the spark gap, means coupled to the spark gap circuit for deriving an output pulse for each gap discharge pulse having a breakdown delay time that is shorter than a reference time period which defines the transistion between a normal spark discharge pulse and an arc discharge, means for counting the output pulses, and means for comparing the counted pulses with a reference number to produce an output signal when the count exceeds the reference number.

6. Apparatus as claimed in claim 9 further comprising second means responsive to the gap discharge pulses for deriving a reset signal when the breakdown 8. Apparatus as claimed in claim 9 wherein said deriving means comprises, means for detecting an electric current across the gap for each gap discharge pulse, a first pulse generator responsive to said detecting means for generating a first sequence of pulses in synchronism with the gap discharge pulses, a second pulse generator triggered by the trailing edge of the pulses generated by the first pulse generator for generating a second sequence of pulses, a delay element for extending the width of the pulses of said second pulse sequence, and gating means responsive jointly to the delayed pulses of the second pulse sequence and the undelayed pulses of said first pulse sequence.

9. Apparatus as claimed in claim 12 wherein the output of said gating means is coupled to an input of said counting means for supplying thereto said output pulses, said deriving means further comprises means for inverting said delayed and undelayed pulses, second gating means responsive to the inverted pulses to derive a reset signal when the breakdown delay time of a discharge pulse is greater than the reference time period, and means for applying the reset signal to an input of said counting means to reset same upon the occurrence of each reset signal.

10. Apparatus as claimed in claim 9 wherein said deriving means comprises, means for producing a first sequence of pulses that correspond in time to the occurrence of the gap discharge pulses, means for producing a second sequence of pulses that correspond in time to the time period between successive discharge pulses, a delay element which extends the duration of the pulses of said second pulse sequence for a period equal to the reference time period, and means responsive to the delayed pulses of the second pulse sequence and the undelayed pulses of said first pulse sequence to derive said output pulse.

11. Apparatus as claimed in claim 14 wherein said selective connecting means comprises a controlled switching element connected in series with the spark gap across said supply source, and means for coupling the output of said second pulse producing means to a control electrode of the switching element.

12. Apparatus as claimed in claim 15 further comprising means for coupling said output signal to said control electrode to control the on-off period of the switching element so as to extend the off period when said output signal is produced. 

1. A spark erosion apparatus with means for detecting an arc between a workpiece and an electrode during a spark erosion process comprising, a source of supply voltage connected across the spark gap formed between the workpiece and electrode for producing electric spark discharges across said gap, means for effectively comparing the breakdown delay time of each gap electric discharge with a reference signal having a reference time period that defines the transition between a normal spark discharge and an arc discharge to produce an output pulse for each gap discharge with a breakdown delay time shorter than said reference time period, means for counting the number of consecutive output pulses corresponding to gap discharges having breakdown delay times shorter than the reference time period, and means for comparing the number of pulses counted with a reference number to produce an output signal when the count exceeds the reference number.
 2. Apparatus as claimed in claim 5 wherein the means for comparing the breakdown delay time comprises a logic gate having first and second inputs, first means for deriving a first sequence of pulses with a pulse width that corresponds to the time interval between two successive gap discharge pulses supplied by the supply source, second means for deriving a second sequence of pulses with a pulse width corresponding to the time interval during which an electric discharge occurs across the spark gap, a delay element for extending the width of the pulses of said first pulse sequence, and means for applying the delayed first pulse sequence to the gate first input and the second pulse sequence to the gate second input.
 3. Apparatus as claimed in claim 6 further comprising means jointly responsive to the delayed first pulses and the second pulse sequence for deriving a reset signal when the breakdown delay time is greater than the reference time period, and means for applying said reset signal to a reset input of the counting means.
 4. Apparatus as claimed in claim 7 wherein said reset signal derving means comprises a second logic gate with first and second inputs, first and second pulse polarity inverter circuits interposed in circuit between the output of the delay element and the second gate first input and between the output of said second pulse derving means and the second gate second input, respectively, and means for coupling the output of the second logic gate to said counting means reset input.
 5. Spark erosion apparatus with means for detecting an arc in the gap formed between a workpiece and an electrode comprising, a source of supply voltage for producing electric spark discharge pulses across the spark gap, means for selectively connecting the supply source across the spark gap, means coupled to the spark gap circuit for deriving an output pulse for each gap discharge pulse having a breakdown delay time that is shorter than a reference time period which defines the transistion between a normal spark discharge pulse and an arc discharge, means for counting the output pulses, and means for comparing the counted pulses with a reference number to produce an output signal when the count exceeds the reference number.
 6. Apparatus as claimed in claim 9 further comprising second means responsive to the gap discharge pulses for derving a reset signal when the breakdown delay time of a discharge pulse is greater than the reference time period, and means for applying the reset signal to an input of said counting means to reset same upon the occurrence of each reset signal.
 7. Apparatus as claimed in claim 10 wherein said output signal is coupled to a control input of said selective connecting means to cause same to extend the time period between successive gap discharge pulses whenever said output signal is produced.
 8. Apparatus as claimed in claim 9 wherein said deriving means comprises, means for detecting an electric current across the gap for each gap discharge pulse, a first pulse generator responsive to said detecting means for generating a first sequence of pulses in synchronism with the gap discharge pulses, a second pulse generator triggered by the trailing edge of the pulses generated by the first pulse generator for generating a second sequence of pulses, a delay element for extending the width of the pulses of said second pulse sequence, and gating means responsive jointly to the delayed pulses of the second pulse sequence and the undelayed pulses of said first pulse sequence.
 9. Apparatus as claimed in claim 12 wherein the output of said gating means is coupled to an input of said counting means for supplying thereto said output pulses, said deriving means further comprises means for inverting said delayed and undelayed pulses, second gating means responsive to the inverted pulses to derive a reset signal when the breakdown delay time of a discharge pulse is greater than the reference time period, and means for applying the reset signal to an input of said counting means to reset same upon the occurrence of each reset signal.
 10. Apparatus as claimed in claim 9 wherein said deriving means comprises, means for producing a first sequence of pulses that correspond in time to the occurrence of the gap discharge pulses, means for producing a second sequence of pulses that correspond in time to the time period between successive discharge pulses, a delay element which extends the duration of the pulses of said second pulse sequence for a period equal to the reference time period, and means responsive to the delayed pulses of the second pulse sequence and the undelayed pulses of said first pulse sequence to derive said output pulse.
 11. Apparatus as claimed in claim 14 wherein said selective connecting means comprises a controlled switching element connected in series with the spark gap across said supply source, and means for coupling the output of said second pulse producing means to a control electrode of the switching element.
 12. Apparatus as claimed in claim 15 further comprising means for coupling said output signal to said control electrode to control the on-off period of the switching element so as to extend the off period when said output signal is produced. 